existentialism:

• The Sickness Unto Death - Søren Kierkegaard
• Nausea - Jean-Paul Sartre
• Waiting for Godot - Samuel Beckett
• Notes From Underground - Fyodor Dostoyevsky
• Being and Nothingness - Jean-Paul Sartre
• No Exit - Jean-Paul Sartre

ethics:

• The Ethics of Ambiguity - Simone de Beauvoir
• A Treatise of Human Nature - David Hume
• Groundwork of the Metaphysics of Morals - Immanuel Kant
• The Nicomachean Ethics - Aristotle
• What We Owe to Each Other - T. M. Scanlon
• Ethics Without Principles - Jonathan Dancy

absurdism:

• The Myth of Sisyphus - Albert Camus
• The Stranger - Albert Camus